Optical and capacitance type, phase transition, humidity-responsive devices

ABSTRACT

The invention provides novel chemical compositions for use in primary, optical/capacitance hygrometric devices. It also provides methods for using these compositions as sensors for the precise measurements of the humidity of gases as well as the apparatus. The chemical compositions, which of themselves sense the change in water vapor pressure, are birefringent, translucent, and anisotropic at a first water vapor pressure/temperature, but non-birefringent, optically clear, and isotropic at a second vapor pressure/temperature. Optical changes which accompany these phase changes may be amplified. Some compositions, exhibit abrupt and large increases in their dielectric constant coincident with the optical changes which occur at the phase shift point. Others show a large and precisely linear change in dielectric constant over many decades of relative humidity (RH), but the optical shift occurs at a precise point within the range. By noting the capacitance readout from a prestandardized combination optical/capacitance hygrometric device at the moment of optically indicated phase shift (when the system changes from isotropic to birefringent), any drift in the electronics of the device can be immediately detected since the sensor&#39;s phase shift point is invariant. If it is desired to eliminate polarizer amplifiers because of their cost, sensor substrates can be shaped so that internal reflection within the substrate and optical coupling of the birefringent sensor to the substrate generate excellent optical signals at the sensor&#39;s trigger points. The devices and methods of the new hygrometry allow many variations on the primary requirements. If desire, the sensor can be made an integral part of a capacitance-sensitive IC.

This is a continuation patent application of U.S. Ser. No. 07/078,186filed Jul. 27, 1987.

BACKGROUND OF THE INVENTION

In my pending patent application, U.S. Ser. No. 763,003, I have notedthat scores of hygrometric devices have been developed in response to aworld wide need among modern societies for the indication and control ofthe humidity of myriad processes and locations in commerce, industry,and the sciences. I particularly stressed that nearly all of thesedevices have been "secondary" types which depend on non-reproducibleprocesses such as moisture sorption by various materials. As a result,the devices have no inherent accuracy, they drift badly over a period oftime, and they are not suitable for myriad demanding needs.

In my pending application I noted that only a few inherently accurate"primary" type devices based on unvarying physical phenomena have beendeveloped. These have been characterized by high cost, high powerconsumption, and large sensor bulk and equipment size. Heading the shortlist of primary devices has been the "dew temperature" or "cold mirror"hygrometer.

My pending application describes a new type of primary opticalhygrometer in which the mirror is heated instead of chilled. It isanalogous to the cold mirror hygrometer but it is highly superiorbecause of its smaller size and lower power consumption. However, inspite of its general excellence for many applications, neither it northe cold mirror hygrometer is suited for the overallultra-miniaturization found in ICs (integrated circuits) or VLSICs (verylarge scale integrated circuits). This is because substantial butvariable cooling or heating of the integrated circuits which areintimately associated with the operation of the sensor mirror introducescomplex non-linearity problems and radiation effects.

In response to the urgent need for miniature military radiosondes,humidity sensors to incorporate within VLSIC "packages," and othercritical applications, humidity sensors have been developed by others.These depend on the change of electrical capacitance of variousdielectric materials as the humidity over the materials varies. However,these sensors are of the secondary type in which the moisture (whichcauses the dielectric constant change in the dielectric material) issorbed in a non-linear and non-reproducible way which shifts with time.Thus, if the sensor is accessible (as in a radiosonde) a very difficultfield standardization of the sensor must be attempted before sending theballoon aloft. If the sensor is sealed into a VLSIC package to monitorits interior, there is no way to check and compensate for the sensor'sdrift. Thus, what has been sorely needed is a capacitance-type, primarystandard humidity device which (a) operates at ambient temperature, (b)has a sensor which is invariant, and (c) has a mode for quickly checkingfor electronic circuit problems.

The sensors of the present invention are unusual in that thehumidity-responsive sensors generate concurrent primary optical-type andprimary capacitance-type signals. Even well designed and properlymanufactured integrated circuits can abruptly develop circuit problems.These can result in false humidity readouts even though thecapacitance/humidity changes of these sensors are invariant. With thesenew sensors an ongoing monitoring of the soundness of the electronics ofdevices which utilize the snesors in the capacitance mode can be readilyprovided by simultaneously optically monitoring the sensor. For example,providing a small window in one of the capacitance electrodes will allowthe sensor film to be seen. It will show a dark field/bright field shiftevery time the gases over the sensor reach a known, precise, invarianthumidity. A readout of this humidity should appear concurrently, ofcourse, on the display screen of the capacitance meter. If desired, forconvenience the same sensor material as is used between the capacitanceelectrodes can be optically displayed elsewhere in the systemm whichtransports the gas across the capacitance electrodes/sensor film formeasurement.

The use of spatially separated dual sensors of the same composition, onesensing optically and the other capacitively, has the additional meritof allowing one to readily optimize the thickness of each for itsparticular function. Thus, an optical-type, humidity-/responsive sensormight be coated at a thickness of about 0.01 mm. in order to secure abrilliant readout. However, a capacitance-type sensor might be coatedmore thinly since the capacitance and the sensitivity of an"electrode/humidity-responsive dielectric composition/electrode" triadincreases, as the thickness of the dielectric composition decreases. Ifdual sensors are not convenient, a very small area of a very thinlycoated sensor film being monitored capacitively may be of greaterthickness so that when scanned optically a brilliant optical readoutresults.

In my U.S. Pat. Nos. 3,776,038, 4,166,891 and 4,175,207 I describeoptical type, humidity-responsive devices of great utility which coverthe middle range of relative humidity. However, in computer science,electronics in general and other specialized fields, the extremehumidity ranges are of special importance because of the corrosiveeffect of water condensing in equipment at very high humidities andstatic sparks (electrostatic discharges) damaging microelectronics atvery low humidities. Both in electronic capacitive-responsive devicesand in direct optical readout devices the sensors of the new technologyallow covering the relative humidity (RH) range down to 15% and belowand 85% and above. Thus, their primary, invariant qualities allowprecise monitoring of ranges which until now have been notorious forgenerating badly drifting signals since the sorption processes used byordinary hygrometers become increasingly erratic at very high and verylow humidities.

SUMMARY OF THE INVENTION

The present invention provides novel chemical compositions for use inprimary standard, optical/capacitance-type or optical-type orcapacitance-type hygrometric devices. It also provides methods for usingthese compositions as sensors for the precise measurement of thehumidity of gases as well as the apparatus.

As a result of extensive research I have discovered novel chemicalcompounds which function in a new and unexpected way to indicate theambient humidity through changes in optical properties and/or electricalcapacitance. I have also discovered what I term "lattice modifiers",novel compounds to extend the humidity ranges covered by the basicsensors. Further, I have devised compounding methods for the sensors andsensors/lattice modifiers which make them suitable for productioncoating and utilization in a variety of hygrometers, controls, andhumidity-sensing devices.

The devices and methods of the new hygrometry allow many variations onthe primary requirements. For example, if the sensors are to be operatedin the optical mode, light sources such as LEDs, tungsten lamps, neonlamps, etc. can be used with a variety of light detecting means such asphoto-transistors, phoyto-SCRs, photo-diodes, photo-resistor cells, thehuman eye, etc. If amplification of the optical change is desired,circular-type polarizers can be used with reflection systems andcrossed-linear type polarizers can be used with transmission systems. Ifthe sensors are to be operated in the electrical capacitance mode,electrical power of various voltages can be used. Frequencies used mayrange from direct current to radio frequencies, though I have found a3-10 volt signal at 1000 Hz a convenient source. Various capacitancemeasuring devices of good accuracy are readily available, the meterswidely used for measuring the capacitance of capacitors proving quitesuitable. If desired, the sensor can be made an integral part of acapacitance-sensitive IC. Thus, a humidity-induced change in thedielectric constant of the sensor of the IC device produces aproportional change in the output of the IC device.

The sensors used for capacitance mode sensing can be of widely varyingarea and thickness depending on the electronics used for indicatingand/or recording and the applications of the device. Electrode spacingcan be varied, and electrode materials of various types can be used solong as they are not corroded by the humid gases being sampled or by thesensor compounds.

A number of modes of placing electrodes have been used by workers inthis field and most of these work nicely. These include co-planarinterdigitated electrodes, parallel plate electrodes, and co-planarseries connected electrodes. Opposed, planar electrodes, with the upperelectrode perforated so that the monitored gas reaches the sensor filmwhich is coated onto the lower electrode work well. To preventelectrical conduction through the sensor compound, the upper or lowerelectrode can be coated with a very thin film of insulating polymer, oran air space can be left between the sensor film and the upper,perforated electrode.

The use of co-planar, polymer insulated, interdigitated electrodescoated with the humidity sensing material also can be used. Often, a gaspermeable, light weight secondary electrode is floated on the surface ofthe sensor film over the interdigitated electrodes so as to secure ahigher electrode area exposure than can be secured from only the edgesof interdigitated electrodes. If desired, two or more sensor filmscovering contiguous humidity ranges can be applied to the electrode in acoplanar, contiguous manner so as to cover an exceptionally broadhumidity range. Or, an electrode bearing a sensor film covering onehumidity range can be covered with a humidity permeable,diffusion-blocking polymer film and then coated with a sensor filmcovering a contiguous humidity range. The second electrode is thenlocated appropriately above the multiple-sensor layer film.

The chemical compositions, which of themselves sense the change in watervapor pressure, are birefringent, translucent, and anisotropic at afirst water vapor pressure and temperature, but non-birefringent,optically clear, and isotropic at a second vapor pressure andtemperaturer. As was noted, the optical changes which accompany thesephase changes may be amplified by passing the light beam through apolarizer and passing the polarized beam through the composition andthen through an analyzer to a detector.

Some compositions, when compounded appropriately, exhibit abrupt andlarge increases in their dielectric constant coincident with the opticalchanges which occur at the phase shift point, the middle section of thecurve being substantially linear. Others show a large and preciselylinear change in dielectric constant over many decades of relativehumidity (RH), but the optical shift occurs at a precise point withinthe range. Thus, though non-linearity may be encountered at the veryends of such curves, the linear sections are very large and so arereadily used for effective commercial and scientific read-out andcontrol. By noting the capacitance readout from a prestandardizedcombination optical/capacitance hygrometric device at the moment ofoptically indicated phase shift (when the system changes from isotropicto birefringene), any drift in the electronics of the device can beimmediately detected since the sensor's phase shift point is invariant.

If it is desired to eliminate polarizer amplifiers because of theircost, sensor substrates can be shaped so that internal reflection withinthe substrate and optical coupling of the birefringent sensor to thesubstrate generate excellent optical signals at the sensor's triggerpoints.

THE DRAWINGS

FIG. 1 is a vapor pressure curve for hydrates of a typical salt, MA(where M is a metal cation and A is an acid radical), at 20° C.

FIG. 2 shows typical vapor pressure temperature curves for water andhydrates of a salt, MA.

FIG. 3 is a schematic view of an optical-type transmission system.

FIG. 4 is a schematic view of an optical-type reflection system.

FIG. 5 is a schematic view of an optical-type transmission system usingsequential RH series plaques.

FIG. 6 is a section of a capacitance-type humidity sensing device.

FIG. 7 is a section of an optical-type reflection system not requiringpolarizer amplification.

FIG. 8a is a capacitance vs. RH curve for an alkali metal complexhumidity sensor much as claimed in U.S. Pat. No. 4,166,891.

FIG. 8b is a capacitance vs. RH curve for the huidity sensor cesiumbenzoylpropionate.

FIG. 8c are capacitance vs. RH curves before and after thermalprocessing for a humidity sensor comprising calcium chloride compoundedwith silica; and

FIG. 8d is an enlarged section of the capacitance vs. RH curves of FIG.8c.

DETAILED DESCRIPTION OF THE INVENTION

In my U.S. Pat. Nos. 3,776,038, 4,166,891, and 4,175,207 I claim sensorcompositions which undergo two phase shifts, namely, from an isotropicsolid to an anisotropic solid and then, at higher humidities, from ananisotropic solid to an isotropic solution (usually used in a gelledstate). This can be summarized thus: ##STR1##

Though this dual signalling has worked well for direct readout devices,electronic devices can be more simply designed if the sensor undergoesonly an unambiguous "on/off" or "birefringent/non/-birefringent" shift.The novel sensors of the present inventoin undergo only phase shift (2)shown above. This is a reversible shift between an anisotropic(birefringent) crystal and an isotropic solution (non-birefringent).

These mechanisms can be described simply with reference to FIG. 1. If weincrease the vapor pressure of water in the presence of the sensor saltMA, no reaction occurs until a critical vapor pressure is reached. Thena hydrate, MA.sH₂ O, with s representing a specific molecular ratio,starts to form. Any attempt to increase the water vapor pressure is thencountered by the formation of more hydrate until all of the MA hasreacted. Only then can the water vapor pressure increase. Because of therelatively easy removal or addition of water, the hydrate water isspecifically indicated as H₂ O, though it is chemically bound.

If the water vapor pressure increases still more, the hydratedeliquesces. That is, the compound removes water from the surroundingatmosphere to form an isotropic solution. All soluble salts aredeliquescent when the partial pressure of the water vapor in theatmosphere exceeds the water vapor pressure of their saturated solution.Though the birefringent, anisotropic sensor salt in the phase equilibriashown above (and illustrated in FIG. 1) is hydrated (MA.sH₂ O),birefringent salts can also be anhydrous. In such a case, the anhydrousbirefrigent salt directly deliquesces to a non-birefringent, isotropicsolution. Once all of the salt has dissolved, a further increase ofwater vapor pressure brings dilution of the saturated solution. Thistechnology uses the crystal/solution phase shift point for opticalsignalling because it occurs at a very precise and reproducible humidityand temperature. The salts may be gelled so that the sensor passes fromopalescent, anisotropic films to clear, isotropic gels at the opticalsignalling point.

FIG. 2 shows the effect of temperature on the equilibrium of the systemdescribed above. On each line the phases stable in the adjacent regionsare in equilibrium. The stable crystal or liquid phases are: Below I,anhydrous; I-II, MA.sH₂ O; II-III, solution.

As noted earlier, the sensors of this technology undergo both opticaland dielectric constant changes of great potential utility. Thesechanged may be utilized separately or together. Utilizing opticalchanges only will be considered first. As notes, this technology usessensing compounds which undergo isotropic/anisotropic changes at thephase shift point or "trigger" point. In the isotropic form the opticalproperties of the solution are the same in every direction. The phaseshift forms anisotropic crystals which have optical properties whichvary with direction. In particular, the crystals have at least twoindices of refraction and so are birefringent. Further, the shift fromthe clarity of the liquid phase to the opalescence of the crystal phasemay be amplified readily with polarizers.

In a representative transmission-type embodiment of the optical mode ofthe present invention shown in FIG. 3, a light beam from light source 5which may be a window, a tungsten lamp, an LED, etc. passes throughpolarizer 1 where the light beam is polarized. The beam then passesthrough transparent or translucent substrate 2 which may be glass orsome isotropic plastic such as cellulose triacetate, on which isdeposited sensor layer 3. The beam passing through 2 and 3 thenencounters analyzer 4 whose polarizing axis is usually at right anglesto the polarizing axis of 1 so as to result in what is generally termed"dark field."

If the coating 3 on substrate 2 is in its non-birefringent mode, littlelight passes through analyzer 4 and the system appears "dark field" toviewer 6. However, if the humidity changes sufficiently, coating 3becomes birefringent. When a light beam enters a birefringent or, as itit is sometimes called, double refracting material, it is divided intotwo components, one defined as an extraordinary ray and the other as anoridnary ray, each vibrating in a direction at right angles to the otherand traversing the birefringent material with a different velocity tothereby introduce a phase difference there between. As said beam istherby resolved into two components, one of which is retarded withrespect to the other, said beam is generally referred to as beingelliptically polarized. The two components emerging from thebirefringent material and entering the second sheet of polarizingmaterial 4 are resolved into one plane-polarized beam again. But a phasedifference has been introduced between the two parts of this same beam,and so the necessary conditions for interference are present. With awhite light source brilliant colors will emerge from analyzer 4 if thecoating consists of large crystals. However, when the crystals are verysmall there is a mixing of colors and the crystal mass appears white.But in either case the field which was previously a blue-black passingvery little light now glows brilliantly.

FIG. 4 typifies a reflection-type system in which light beams from asource 5 pass through a circular-type polarizer 10 where they arecircularly polarized. They then pass through the sensor layer 3 coatedonto substrate 2 to the polarization conserving mirror 9. When thecoating 3 is non-birefringent, no light will be reflected back through10 because the circular polarizer has polarized the beam to a"right-handed" or "left-handed" helix form which cannot pass backthrough the circular polarizer 10. When coating 3 becomes birefringent,the polarization form of the light that is reflected from the mirror isaltered and the returning light passes through the polarizer 10.

FIG. 5 also represents a transmission-type embodiment of the opticalmode but it is one in which a graded series of RH sensors are displayedso as to cover a particular humidity range. It is similar to FIG. 3 butinstead of a single coating 3 on substrate 2, the graded series 3, 3'3", etc. are applied in discrete areas on the substrate.

FIg. 6 represents a section of a typical embodiment of thecapacitance/optical mode of the technology. 7 is a side slotted, annularholder made of an electrically insulating polymer of low water sorptioncharacter such as Teflon into which a threaded retaining ring of Teflon12 screws. 9 is a flat, polished circular metal mirror electrode coatedon the upper mirror side 9a with a thin, isotropic electricallyinsulating film such as the insulating polymer heretofore referred to oran aluminum oxide as is identified in FIG. 6 at 9b placed upon the base7a of holder 7 and which has a long narrow tab 9c extending through theside of slotted holder 7. 3 is a humidity-responsive sensor coating ofthe present invention coated on the middle of the upper surface 9d ofmirror 9. 8 is an annular spacing ring of an electrically insulating,low water sorption polymer such as Teflon machined to a precisethickness. 11 is a flat, perforated, circular, metal electrode having along, narrow tab 11a (similar to that of 9) which extends thorugh theside of the slotted holder 7. To monitor the state of the sensor film, alight beam from light source 5 passes through circular-type polarizer 10where the beam is circularly polarized. Then the beam passes through thecircular aperture 13 in perforated metal electrode 11 and then, as inFIG. 4, through the sensor layer 3 coated onto mirror substrate 9. Asexplained before, reflected light from light mirror 9 emerges from theaperture 13 and passes through the polarizer 10 to be perceived by theviewer or by a photo-responsive device at the moment the sensor layer 3becomes birefringent. Electrodes 9 and 11, of course, are connected to asuitable capacitance meter 14 which may be calibrated in percent RHrather than picofarads. Voltage is applied to the electrodes by voltagesource 15. Prior determination of the primary, inherent "trigger point"or optical phase change point of the sensor material used allows precisechecking against the readout shown by the capacitance meter at themoment of optical shift. Perforated electrode 11 may have its viewingport and ventilation holes closed with a very thin coating of a polymerhighly permeable to water vapor (such as cellulose acetate butyrate) ifit is desired that the cell be impermeable to liquid water. In such acase the emerging electrode tabs are sealed with an RTV siliconecompound. A polymer coating 0.002 mm. or less is suitable on the topelectrode.

FIG. 7 is a cross section of a trapezoid-shaped bar of transparent glassor of a plastic such as polymethyl methacrylate. This has beendiscovered to generate excellent optical signals without the use ofpolarizer amplifiers when used with the humidity sensors of the presentinvention. The device consists of a bar shaped substrate 2 of suitablelength to carry one or more plaques of sensor coating 3 on its backside. Light beams form light source 5 enter the trapezoidally shaped barwhose angles have been chosen to cause total reflection to occur to thebeams which encounter the two non-parallel sides 2a, 2b of thetrapezoid. To illustrate this reflection, beams 5 are shown entering thestructure, being internaly reflected, and exiting as beam 5'. Since thetrapezoid is symmetrical, this reflection occurs on both sides.

Unexpectedly, the sensors of the present invention have been found tooptically couple with the trapezoidally-shaped substrate when thesensors are in their birefringent, opalescent phase. The viewer 6perceives only a faint, opalescence when looking through the bar 2 andthe coating 3 in the center zone (between the lines of beams 5 and 5').However, the two edge zones, which are mirror reflections of coating 3,appear brilliantly white. When sensor coating 3 shifts to isotropic andtransparent, the two edge zones reflecting that sensor abruptly becomesilver, giving a dramatic signal shift.

Many variations of this structure are possible. The front center zonecan be used to carry a scale showing the trigger humidities of a gradedseries of sensor plaques on the back side. Instead of a trapezoidalshape, a semi-circular or even half of an annular shape can be used. Inthe case of the trapezoidal shapes, it is necessary to select trapezoidangles which will cause total internal reflection within the particulartransparent material used. The "critical angle" can be readilycalculated from the index of refraction of the desired substrate, ofcourse. Light beams also can be passed into either end of such bars toilluminate sensors coated on the back. If desired, the readout fromthese systems, can be further enhanced by viewing the front face througha circular-type polarizer parallel with it.

If a trapezoidal shape is used, the humidity-sensing compositions areapplied only to the flat back of the device, in one or more stripes ofany desirable width, optimally extending from one edge to the otherwhatever the width selected for the back of the trapezoid. Though suchstripes can be carried up the non-parallel sides of the trapezoid, onlya very small increase in signal contrast results and the effort is notwarranted.

If a semicircular or semiannular shape is coated with one or morehumidity-sensing compositions, the stripes as before are parallel and ofany convenient width. For convenience in coating, the stripes may extendfrom one front edge to the other front edge, spanning the entire 180° ofthe semicircle. However, the length of the stripe may be substantiallyshortened while still generating a good optical signal. Generally, it isdesirable that at least 25% of the semicircular arc be coated and thecoating should be of approximately equal length either side of thecenterline.

FIG. 8a is a capacitance vs. RH curve of a sodium/potassium mixedcrystal salt complex such as is described in claim 13 of my U.S. Pat.No. 4,166,891. It has an optical trigger point of 50% RH (indicated bythe vertical dashed line at 50%). Magnesium compounds are an essentalstabilizing component in this complex of the alkali metal salts of3,3',4,4' benzophenonetetracarboxylic acid. As noted elsewhere,magnesium compounds cannot be used in conjunction wth the complexing ofthe alkali metal organic salts of the present invention since they causedecomposition. The huge shift in the dielectric constant of the complex,as indicated by the almost vertical shape of the capacitance vs. RHcurve, occurs concurrently at the optical trigger point. This curve wasdeveloped using the sensor fixture illustrated in FIG. 6, as were theother curves shown in the FIG. 8 series.

FIG. 8b is a capacitance vs RH curve of the pure organic salt, cesiumbenzoylpropionate, whose preparation is described in detail further on.In this typical example, the large, linear change in the dielectricconstant extends over almost five decades of relative humidity asindicated by the vertical, dashed lines. Homologs of this compoundextend the range covered to very high humidities and lattice modifiersextend the range to very low humidities. As noted elsewhere, the opticalshift for each compound occurs at a very precise humidity along itscapacitance vs. humidity curve.

FIG. 8c shows capacitance vs. RH curves of the inorganic salt, calciumchloride, compounded with a high surface area silica. As a matter ofinterest, curve 1 shows both the high RH as well as the low RH portionsof the curve. However, as explained in detail further on, this sensor isan example of a special type used for monitoring low humidities undervery rigorous conditions. Thus, curve 2, which is a capacitance vs. RHcurve developed with the identical sensor after a 500° C. thermalporcessing, was tested only at lower humidities since this is the rangeof interest.

The vertical dashed line marks the 2000 parts per million by volume ofwater vapor which the Military has established as the limit of internalwater vapor content for high reliability microcircuits packaged inhermetically sealed ceramic packages. This is equivalent toapproximately 8% RH at 20° C. and it is in this lower range that asensor such as this is used for monitoring the atmosphere of sealedpackages.

FIG. 8d shows the same two capacitances vs. RH curves 1 and 2 as shownin FIG. 8c, but the scales are expanded to cover only the capacitanceand RH ranges of interest. It shows the capacitance readouts which canbe readily secured in greater detail than can be seen in the curves ofFIG. 8c.

Curves 1 and 2, FIG. 8d, vary by about 1.5% RH since, as explainedelsewhere, the test assembly shown in FIG. 6 was not designed to befired at 500° C. In particular, after developing curve 1, the assemblywas disassembled, the electrode bearing the sensor film was fired at500° C., cooled, and reinstalled in the assembly for determining curve2. Capacitance test devices designed to be assembled and disassembledare manufactured to very close tolerance because of the large change incapacitance with small change in electrode clearance. The test device ofFIG. 6 is not of this precision. However, the FIG. 6 device, onceassembled, has repetitively generated such curves as are shown in FIG.8a and FIG. 8b. Curves such as 1 and 2 of FIG. 8c and FIG. 8d, thoughalready acceptable by present commercial standards, are expected to fallinto very close clusters when sensors such as are described here areinstalled in electrode assemblies designed for high temperatureprocessing.

Turning to the chemical compounds which I have discovered exhibit theseconcurrent optical and dielectric constant changes, I have found themremarkable for their sensitivity to very small changes in the humidityof gases in contact with them. This is believed to be due to theunusually propitious balance of forces within the molecules. Thus, asthe ambient humidity varies and they shift phase from an isotropicsolution to anisotropic crystals, they do not form the highly organized,tightly bound type of crystal which dissolves with difficulty as thehumidity varies. Nevertheless, the intermolecular forces among thedissolved molecules are strong enough to eliminate any tendency towardsupersaturation when water leaves the system.

There is an explanation for this remarkable behavior. Though I do notwish to be bound by theory, it is believed that the salts of the presentinvention form lyotropic liquid crystals as solvent water leaves theirisotropic solutions. Though a good deal of work has been done on"thermotropic" liquid crystals (the type used in "liquid crystal"thermometers and electronic "liquid crystal" displays) and which areentirely different, little work has been done with lyotropic liquidcrystals. The common bond between the two categories is that in eachcase the sensing molecules shift between an isotropic liquid state and a"liquid crystal" state. The term "liquid crystal" is used to describe astate in which a good deal of molecular order is present as comparedwith the liquid state. However, the molecules are not as organized asthey are in the solid where constituent molecules execute smallvibrations about firmly fixed lattice positions but cannot rotate. Theordinary liquid state, of course, is characterized by relativelyunhindered molecular rotation and no long-range order.

First turning to the chemistry of these sensors in a general way, thesensory molecules are amphiphilic. That is, they are characterized bypossessing an organic ring structure that is water insoluble(hydrophobic) and a side chain with a polar head (ionic) which dissolvesreadily in water (hydrophilic). However, I theorize that under suitableconditions the cation of the side chain bonds to the oxygen of thebenzoyl group to form a novel polar ring complex which is highlysusceptible to hydration because of its ionic nature. This compactmulti-ring structure which forms is believed to be unusually suitablefor the formation of lyotropic liquid crystals. It is also a structureoffering excellent stability to heat and ultraviolet light.

I have found that a number of molecular structural modifications can bemade and still preserve the essential humidity-response of the sensorsof the present invention. However, the alkali metal salts of3-benzoylpropionic acid, C₆ H₅ COCH₂ CH₂ CO₂ H, or the alkali metalsalts of the ring-substituted benzolypropionic acids have proven ofexceptional utility. The alkali metals which are useful include lithium,sodium, potassium, rubidium, and cesium since the different metals formsalts which trigger or undergo phase shifts at different humidities.Further, so as to prepare formulations which trigger at intermediatehumidities, mixtures or so-called mixed crystals can be formed bycross-mixing the salts of the different alkali metals.

Among the various substituent groups which are useful in modifying thephenyl ring, I have found that alkyl groups create compounds whosealkali metal salts trigger at humidities far higher than do those of thenon-alkylated benzoylpropionic acid. For example, the compoundedpotassium salt of 3-benzoylpropionic acid triggers at about 53% RH at20° C. but the compounded potassium salt of 3-(4-ethylbenzoyl) propionicacid, C₂ H₅ C₆ H₄ COCH₂ CH₂ CO₂ H, triggers at 83% RH at 20° C. Agreater degree of alkylation raises the trigger point of the alkalimetal salts still higher.

The lowest humidity which the compounded (but non-modified) alkali metalsalts of 3-benzoylpropionic acid will sense is 35% RH at 20° C., thetrigger point of the cesium salt. However, it is desirable to sense muchlower humidities. I have discovered a new class of compounds which meetthis need, and I term them "lattice modifiers" since they are believedto function by profoundly altering the intermolecular forces within theliquid crystal lattice. Thus, the prime, sensing molecules (for examplethose of cesium benzoylpropionate) do not associate at their "natural"trigger humidities but at much lower humidities when mixed with theselattice modifiers.

These modifiers are chain-like with polar heads (ionic) and they closelyresemble the side chain attached to the phenyl group of prime sensingsalts such as cesium benzoylpropionate, C₆ H₅ COCH₂ CH₂ CO₂ Cs, in which--COCH₂ CH₂ CO₂ Cs is the side chain. For example, an effective latticemodifier, cesium hydroxybutyrate, H₂ COHCH₂ CH₂ CO₂ Cs, is the same ascesium benzoylpropionate's side chain except that the keto group (C═O)has been replaced by an hydroxyl group (C--OH). As before, it isbelieved that the chain forms a ring structure in which the Cs bonds tothe oxygen of the hydroxyl group. It is theorized that these molecular"separators" or spacers make it increasingly difficult for the primesensing molecules to assemble into liquid crystal signalling "swarms" orarrays. The reduction of the RH trigger point is a linear function ofthe quantity of lattice modifier added, and the modifiers are quitepotent. By such a method the trigger point can be reduced below 15% RHat 20° C. while preserving the full brilliance of readout and thesensitivity to small humidity changes.

What I term "compounding" of the prime sensor salts and their latticemodifiers is also of great value, though for many applications thesensor salts (with or without modifiers) can be used as uncompoundedsolutions. When they are used in an uncompounded form, it is desirableto limit the area and depth of each incremental sensor salt deposit sothat surface tension prevents running or draining of the deliquescedliquid which it forms at high relative humidities. This is especiallytrue if the sensors are to be used in the vertical position. Then thesolution can be applied in small droplets. If desired, the surfacetension of the substrate area between droplets can be readily raised bycrosshatching the substrate with narrow line deposits of water repellentmaterials such as fluoropolymers.

Compounding of the sensor salts offers various benefits. For example,equalizing the viscosity and surface tension of 16 or more sensorsolutions before simultaneously coating a graded RH series onto a movingsubstrate greatly facilitaties the process. Once applied, propercompounding creates chemical complexes which signal brilliantly,shifting from a birefringent liquid crystal state to an isotropic gelledstate, a form which never becomes fluid enough to drip or run at highhumidities.

In my U.S. Pat. No. 4,166,891 I describe polymers and certain metals andborates which create complexes of great utility when used with thosechemical sensors. In the claims covering those complexes I particularlyspecify magnesium, Mg⁺⁺, because it is essential to the long termstability of those complexes. However, I have discovered that thechemistry of the present sensor compounds is entirely different fromthat encountered with the alkali salts of 3,3'4,4' benzophenonetetracarboxylate, the sensors of the earlier invention. With the newsensors the inclusion of magnesium salts brings immediate formation ofinsoluble complexes which make the composition useless. Thus, thoughthere appear to be similarities between the complexing of thebenzophenone sensors and the salts of the present invention, theessential chemistry of the complexing is entirely different as isconfirmed by the magnesium reaction.

As before, I have found that water soluble, organic polymers containingrepetitive oxygen-bearing groups including the hydroxyl, the carboxyl,the sulfonic acid group, or mixtures of them repetitively present alonga substantially linear chain are useful compounding ingredients.Examples of these polymers, which are preferably solid and which haveproven effective, are as follows: carboxyl group-poly(methyl vinylether/maleic anhydride), poly(styrene/maleic anhydride), polyacrylicacid; sulfonic acid group-polyvinyl sulfonic acid; methoxy group-methoxycellulose; polyether group-polyethylene oxide; polyamidegroup-poly(vinylpyrrolidone). The carboxyl and sulfonic acid groups mustbe neutralized with an appropriate alkali metal compound before use sothat the system is neutral or slightly basic. Some of these polymers aremodified in various ways during manufacture. For example, polyacrylicacid is often modified by inclusion of methacrylic acid and/orcrosslinking agents during manufacture.

The molecular weights of the polymers in general are not critical solong as the polymers form stable, water solutions of appropriateviscosity. Usually, from about 2% to 8% of the polymer in a coatingsolution containing 8% by weight of sensor salt contributes goodrheological properties to the coating solution.

Though I do not wish to be bound by theory, it is thought that the metalcations which I have found effective with the present chemical sensorscoordinate with the oxygen of the polymers to form elaborate networkstructures. The metal ions which coordinate with the polymers and withthe sensors of the new invention to form appropriately complexedcompositions are Cu⁺⁺, Ni⁺⁺, Co⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Zn⁺⁺, and Cr⁺⁺⁺. Sincethe salts must be appropriately soluble for reaction, the nitrate,chloride, acetate, and sulfate typify useful anions. Usually about 5 molpercent of the metals present in an alkali metalbenzoylpropionate/polyvalent metal salt mixture must be of thepolyvalent metal salt for good complexing, though as little as 0.4 molpercent may prove adequate.

Besides the heavy metal cations which form such useful networks with thepolymers to enhance viscosity in the solution and in the deliquescedfilm, I have found that the boron introduced as a borate is a mosteffective agent to gel the viscous films which form on evaporation ofwater from a sensor salt/polymer/polyvalent heavy metal salt solution.Alkali metal borates form complex ring structures in water, and it isbelieved that the oxygenated polymers and polyvalent cations react withthese rings to form elaborate stable networks which can readily swell athigh humidities but which are so crosslinked that they cannot dissolve.

I have found that about two mol percent of borate ion on the basis ofthe metals present in the sensor salts/polyvalent metal salts/polymersblend usually prevents any dripping even at 90% RH and 50° C. However,as little as 0.25 mol percent can prove adequate and as much as 10 molpercent can prove desirable when very high humidities are expected. Thestrength of the complexing of these components, namely, the alkali metalsensor salts and borates, the heavy metal salts, and the polymers isdemonstrated by the brilliantly clear solutions which form while at thesame time the non-flowing films which they form retain their integrityat high temperatures and humidities.

Finely divided inorganic compounds having little or no birefringence andpossessing a high surface area also can be used advantageously incompounding the prime sensor salts and their lattice and modifiers.Since the dielectric constants of the inorganic compounds are quitedifferent from those of organic polymers which are also most useful incompounding, they offer a most convenient mode of modifying theelectrical characteristics of the sensor system while securingrheological properties in the sensor solutions which make them suitablefor successful coating.

Precipitated and pyrogenic silica, diatomaceous earth, and pyrogenicaluminum oxide typify oxides which are commercially available havingsurface areas as high as 50-200 sq. meters/gram. From 0.5 to 4.0% byweight of such materials can be readily dispersed in aqueous solutionsof prime sensor salts (containing lattice modifiers where desired) whichcontain from 10-50% of sensor salts by weight. The pH of the suspensionis then adjusted to the 7.5-8.5 range using the appropriate alkali metalhydroxide. Such dispersions coat well and dry to films of greatsensitivity to humidity changes and free of drainage problems.

Pyrogenic aluminum oxide and silica are of exceptional value because byempirically adjusting the ratio of one oxide to the other the viscosityof a particular dispersion can be adjusted over a much wider range thancan be secured with either alone.

I have found that there is still another type of compounding componentwhich is essential for good signalling with lyotropic liquid crystals. Iterm such materials "lattice stabilizers," and they stabilize the liquidso that the liquid crystals form but one type of structure. Solidcrystals often can form more than one well-defined structure, and sothere is often a problem with stabilizing a system so that only thedesired crystal type forms. Liquid crystals, however, are much morecomplex since the intermolecular bonds are weaker. Thus, many subtle,undesirable variations on a particular, desired crystal structure canoccur in an uncontrolled system because of temperature variations,variations in the rate of crystal formations etc. However, I havediscovered certain additives which are believed to create a kind of"reference lattice structure" within the lyotropic liquid crystal systemso that the sensor molecules always assemble into crystals in such a wayas to generate the same optical and dielectric constant readouts. Thisreference lattice structure is invisible but potent in its organizingpower.

One type of lattice stabilizer consists of aryl hydrocarbons of lowvapor pressure which dissolve in the sensor salt/polymer/water system toform organizing complexes of still lower vapor pressure. Examples ofhydrocarbons which perform well include Monsanto's HB40, which is apartially hydrogenated terphenyl which also contains about 40% ofterphenyl. 1,1-di(ortho-xylyl)ethane, 1,2,3, 4 tetramethylbenzene,1,2,4,5 tetramethylbenzene, 1,2,3,5 tetramethylbenzene, 1,2,3,4,5pentamethylbenzene, and cyclohexylbezene are also hydrocarbons whichbring about consistent organization of the liquid crystals, increase thebrightness of optical readout, and minimize hysteresis. Hysteresis isthe difference in the humidity level at which triggering occurs when thehumidity is rising compared to that met when the humidity is falling.Typically, from 1 to 3% of the hydrocarbon on the basis of containedsolids is very effective.

Another type of potent lattice stabilizer consists of certain surfaceactive agents in which a stable fluorocarbon tail, F₃ C(CF₂ CF₂. . .) isattached to a water-solubilizing group, X. The alkali metalperfluoroalkyl sulfonates, carboxylates, and acid phosphates comprisethe compounds which are effective. The perfluoroalkyl group varies inlength among different compounds and manufacturers but typicallyincludes 3 to 8 carbons. A small concentration of agent is usually quiteeffective. From 0.1% to 0.3% on the basis of contained solids is usuallyquite effective.

Having overviewed the various components of these complex but remarkablyeffective and useful systems, it is helpful to consider the componentsin greater detail. Since the salts of benzoylpropionic acid and itshomologs and analogs comprise the central sensors of these systems, thesynthesis of benzoylpropionic acid is most useful to consider.Typically, the various acids from which effective sensor salts can bemade can be synthesized by using the same method or a variation on themethod which would be obvious to one skilled in the art. Usually, avariation in concentration of reactants, temperature of reaction, orchoice of solvent will produce a satisfactory yield of the desired acidin good purity.

The Friedel and Crafts reaction is the one most useful for preparation.The reaction of succinic anhydride with benzene or the desired alkylatedor arylated benzene or substituted benzene in the presence of anhydrousaluminum chloride produces the desired aroyl acid in good yield andpurity. More generally, the Friedel and Crafts reaction between analiphatic dibasic acid anhydride and an aromatic compound results in theformation of an aroyl fatty acid with the aroyl group situated at thelast carbon atom of the aliphatic chain.

PREPARATION OF BENZOYLPROPIONIC ACID

In a 2-l, three-necked, round-bottomed flask fitted with a mechanicalstirrer and two reflux condensers are placed 68 g. (0.68 mole) ofsuccinic anhydride and 350 g. (4.5 moles) of dry, thiophene-freebenzene. The stirrer is started, and 200 g. (1.5 moles) of powdered,anhydrous aluminum chloride is added all at once. Hydrogen chloride isevolved and the mixture becomes hot. It is heated in an oil bath andrefluxed, with continued stirring, for half an hour. The flask is thensurrounded by cold water, and 300 cc. of water is slowly added from adropping funnel inserted in the top of one of the condensers.

After the addition of water to the aluminum chloride complex, 100 cc. ofconcentrated hydrochloric acid (sp. gr. 1.18) is added and the benzeneis removed by steam distillation. The hot mixture is transferred to a2-l beaker, and the 3-benzolypropionic acid separates as a colorless oilwhich soon solidifies. After cooling to 0° C., it is collected, washedwith a cold mixture of 50 cc. of concentrated hydrochloric acid and 150cc. of water, and then with 200 cc. of cold water. The crude acid isdissolved in a solution of 75 g. of anhydrous sodium carbonate in 500cc. of water by boiling for fifteen minutes. The solution is filteredwith suction and the small amount of aluminum hydroxide washed twicewith 50-cc. portions of hot water. Four grams of charcoal is added tothe hot filtrate; the solution is stirred for three to four minutes andthen filtered with suction. The clear, colorless filtrate is transferredto a 2-l beaker, cooled to 50°-60° C., and carefully acidified with 130cc. of concentrated hydrochloric acid. After cooling to 0° C. in anice-salt bath the acid is filtered, washed well with water, driedovernight at room temperature, and finally dried to constant weight at40°-50° C. The yield is 110-115 g. (92-95 percent of the theoreticalamount). It melts at 114°-115° C. and needs no further purification.

Among the more important homologs of 3-benzylpropionic acid which I haveprepared are the following acids and (in parenthesis) the hydrocarbonsfrom which they were prepared:

    ______________________________________                                        3-(4-methylbenzoyl)propionic acid                                                                     (toulene)                                             3-(4-ethylbenzoyl)propionic acid                                                                      (ethylbenzene)                                        3-(4-propylbenzoyl)propionic acid                                                                     (propylbenzene)                                       3-(4-isopropylbenzoyl)propionic acid                                                                  (cumene)                                              3-(4-butylbenzoyl)propionic acid                                                                      (butylbenzene)                                        3-(4-amylbenzoyl)propionic acid                                                                       (amylbenzene)                                         3-(3,4-dimethylbenzoyl)propionic acid                                                                 ( -o-xylene)                                          3-(2,4-dimethylbenzoyl)propionic acid                                                                 ( .sub.--m-xylene)                                    3-(2,5-dimethylbenzoyl)propionic acid                                                                 ( -p-xylene)                                          3-(2,4,6-trimethylbenzoyl)propionic acid                                                              (mesitylene)                                          3-(2,3,4,5-tetramethyl- (1,2,3,4, tetra-                                      benzoyl)propionic acid  methylbenzene)                                        3-(2,3,5,6-tetramethyl- (1,2,4,5 tetra-                                       benzoyl)propionic acid  methylbenzene)                                        3-(2,3,4,5,6-pentamethyl-                                                                             (pentamethyl-                                         benzoyl)propionic acid  benzene)                                              3-(4-phenylbenzoyl)propionic acid                                                                     (biphenyl)                                            3-(4-cyclohexylbenzoyl)propionic acid                                                                 (cyclohexyl-                                                                  benzene)                                              ______________________________________                                    

Among the substituted benzoylpropionic acids which I have used toprepare various salts are 3-(4-bromobenzoyl) propionic acid,3-(4-fluorobenzoyl) propionic acid, 3-(4-chlorobenzoyl) propionic acid,and 3-(4-methoxybenzoyl) propionic acid.

It is thought that the alkali metal salts of benzoylpropionic acid forma 7-member ring structure in which the alkali metal cation binds to theketo group. By varying the length of the aliphatic chain from the threecarbons of the propionic chain, various size rings are believed to beformed. Various size rings are known to be stressed in varying waysdepending on the number, type and position of atoms forming the ring.Usually, the ring with minimal stress is the most stable. In the case ofthe sensors of present invention, in general the 7-member ring formed bythe alkali metal benzoylpropionates is most stable and signals best.

So as to arrive at the most stable and most effective sensors, the saltsof acids possessing chains of varying length have been evaluated. Amongthese acids 4-benzoylbutyric acid, C₆ H₅ CO(CH₂)₃ CO₂ H, benzoylformicacid, C₆ C₅ COCO₂ H, and, of course, 3-benzoylpropionic acid are mostuseful for preparing humidity-sensing salts for standard applications.Acids having longer chain lengths are suitable for specialized sensorsalts.

Introducing other elements in place of carbon in a ring structure isknown to also effect the stresses in the ring. Thus, I have investigatedthe salts of analogs of benzoylpropionic acid in which, for example,nitrogen has replaced carbon. A good example of this is hippuric acid,C₆ H₅ CONHCH₂ CO₂ H, in which the NH replaces one of the CH₂ groups ofthe propionic acid chain. 4-nitrohippuric acid, O₂ NC₆ H₄ CONHCH₂ CO₂ Hhas also been examined. Other compounds of this type but of differingchain lengths, so that rings of varying size are believed to form,include the following:

N-benzoyl-B-alanine, C₆ H₅ CONHCH₂ CH₂ CO₂ H

N-(4-aminobenzoyl)-B-alanine, H₂ NC₆ H₄ CONHCH₂ CH₂ CO₂ H

N-(4-nitrobenzoyl)-B-alanine, O₂ NC₆ H₅ C₆ H₄ CONHCH₂ CH₂ CO₂ H

N-(p-nitrobenzoyl)-6-aminocaproic acid, O₂ NC₆ H₄ CONH(CH₂)₅ CO₂ H

Instead of single rings such as are present in benzene or biphenyl,fused rings such as are found in naphthalene can be reacted in formaroyl compounds. Additional compounds whose salts have been evaluatedinclude the following:

gamma-oxo-1-naphthalenebutyric acid, C₁₀ H₇ COCH₂ CH₂ CO₂ H

gamma-oxo-2-naphthalenebutyric acid, C₁₀ H₇ COCH₂ CH₂ CO₂ H

gamma-oxo-1-pyrenebutyric acid, C₁₆ H₉ COCH₂ CH₂ CO₂ H

The study of the physical chemistry of the lattice modifiers which Ihave discussed before also has added greatly to an understanding ofthese systems. That is because, as I have already noted, the modifiersclosely resemble the side chains attached to the aryl groups of theprime sensing salts. It is believed that the chain of the modifier formsa ring structure in which an alkali metal carbon bonds to an equivalentof the keto group of benzoylpropionic acid. Because of their structuralsimilarity, discoveries in one system assist in the understanding of theother.

Among the acids which have been evaluated as 6-member formers aremalonic acid, HO₂ CCH₂ CO₂ H, and tartronic acid, HO₂ CCH(OH)CO₂ H.Acids whose salts were investigated and which form 7-member ringsinclude the following:

succinic acid, HO₂ CCH₂ CH₂ CO₂ H

levulinic acid, CH₃ COCH₂ CH₂ CO₂ H

ketoglutaric acid, HO₂ CCH₂ CH₂ COCO₂ H

hydroxybutyric acid, HOCH₂ CH₂ CH₂ CO₂ H

succinamic acid, H₂ NCOCH₂ CH₂ CO₂ H

aminobutyric acid, H₂ N(CH₂)₃ CO₂ H

Acids whose salts were worked with and which may form 8-member rings areglutaric acid, HO₂ C(CH₂)₃ CO₂ H, and its fluorinated derivative,perfluoroglutaric acid, HO₂ C(CH₂)₃ CO₂ H. A possible 9-member ringforming salt examined is that of aminocaproic acid, H₂ N(CH₂)₅ CO₂ H.

The alkali metal salts of analogs of these same acids, in which a phenylgroup replaces a hydrogen of one of the methylene groups, also work wellas lattice modifiers. Examples of acids useful for preparing effectivelattice modifiers of this type include the following phenylsuccinicacid, phenylmalonic acid, phenylhydroxybutyric acid, and phenylglutaricacid.

From these interrelated studies has been developed a general formula forthe acids which form the anions of these humidity sensors:

    R-Aryl-CO-X-(CH.sub.2).sub.Y -CO.sub.2 H

where R is hydrogen, halogen, alkyl, alkoxy, or a nitro group.

Aryl is phenyl or a connected ring structure such as a biphenyl or afused ring structure such as naphthalene, acenaphthene, fluorene,anthracene, or pyrene.

X is a nitrogen or a carbon attached to the adjacent methylene witheither a single or double bond.

Y is 0 to 7

In a similar way, there has been developed a general formula for theacids whose alkali metal salts comprise the lattice modifiers whichallow a smooth and essential reduction of the trigger humidities of theprime sensor salts:

    R-CZ-(CH.sub.2).sub.Y -CO.sub.2 H

where R is amino, hydroxyl, carboxyl, or a methyl group.

Z is hydrogen, hydroxyl, or a keto group.

Y is 0 to 4.

I have already given information on the synthesis of the acids which arereacted to form the sensing salts of technology. The preparation of twotypical alkali metal salts useful as humidity sensors in the pure stateor as componenets of compounded materials follows:

PREPARATION OF CESIUM BENZOYLPROPIONATE

72.0 g. of 99% pure 3-benzoylpropionic acid is slurried in 100 cc.distilled water. 97.3 g. of a 4.11 molar, pure cesium hydroxide solutionis added and the temperature raised to 65° C. to obtain completesolution. The solution is filtered through No. 40 filter paper to removedust, etc. The solution is then evaporated on a hot plate to 200 g.,which gives a 2 molar concentration, and it is then bottled.

PREPARATION OF POTASSIUM ETHYLBENZOYLPROPIONATE

83.3 g. of 99% pure 3-(4-ethylbenzolyl) proponic acid is slurred in 100cc. distilled water. 49.1 g. of 8.144 molar, pure potassium hydroxidesolution is added and the temperature is raised to 65° C. to obtaincomplete solution. The solution is filtered through No. 40 filter paperto remove dust, etc. The solution is then evaporated on a hot plate to200 g., which gives a 2 molar concentration, and it is then bottled.

The preparation of the salts which I have termed lattice modifiers isequally straightforward.

PREPARATION OF CESIUM LEVULINATE

46.9 gr. of 99% pure levulinic acid is slurried in 100 cc. distilledwater. 97.3 g. of a 4.22 molar, pure cesium hydroxide solution is addedand the temperature is raised to 65° C. to obtain complete solution. Thesolution is filtered through No. 40 filter paper to remove dust, etc.The solution is then evaporated on a hot plate to 200 g., which gives a2 molar concentration, and it is then bottled.

These materials are readily compounded into highly stable solutionswhich can be readily coated onto a variety of substrates. Thepreparation of a sensor solution of cesium benzoylpropionate, whichtriggers optically at 35% RH at 20° C. (when applied as a film, typifiesthe preparation of sensors usefull for lower RH sensing.

PREPARATION OF A 35% RH SALT SOLUTION

The following materials are weighed together:

120 g. 10% cesium polyacrylate solution

338 g. distilled water

30 g. M/40 zinc acetate solution

12 g. 4% cesium borate

100 g. 2M cesium 3-benzolypropionate

1 g. Monsanto's HB40

Slow speed agitation serves nicely to mix the various major ingredients.However, the HB40 must be dispensed using a high speed mixer.

By dissolving into the compounded sensor salt increasing quantities of alattice modifier such as cesium levulinate, whose preparation has beendescribed before, the trigger point of the blend can be smoothly andlinearly reduced to 15% or less.

The preparation of a sensor solution of potassium 3-(4-ethylbenzoyl)propionate which triggers at 83% RH at 20° C. (when applied as a film)is representative of the preparation of sensors which trigger at higherhumidities. The procedure is essentially the same as that described forthe 35% RH sensor salt solution.

PREPARATION OF A 83% SENSOR SALT SOLUTION

The following materials are weighed together and processed as before:

120 g. 10% potassium polyacrylate solution

338 g.. distilled water

30 g. M/40 zinc acetate solution

12 g. 4% potassium borate

100 g. 2M potassium 3-(4-ethylbenzoyl) propionate

1 g. Monsanto's HB40

Summarizing, acids can be readily synthesized from whichhumidity-responsive alkali metal salts can be easily prepared. Solutionsof such pure salts can be readily coated onto a variety of substrates toproduce primary, non-drifting humidity sensors of great accuracy. Thereadout can be either in optical or in electrical capacitance changes orin both. The humidity range which such salts can cover is large, and byincorporating readily prepared lattice modifier salts as well, the rangeis substantially expanded.

Compounding such pure, prime sensing salts and their lattice modifierswith appropriate polymers, heavy metal salts, borates, and latticestabilizers creates compositions unusually well suited for industrialcoating. Such coated substrates can be used in direct readouthygrometers, electronic hygrometric controls, electronic hygrometers,hygrometric limit alarms, and many other devices.

Turning again to the various hygrometric devices which were describedearlier in a general way, the simple transmission-type sandwich of FIG.3 represents an excellent answer to the need for a humidity alarm forthe myriad rooms in which are stored goods of value which can be damagedby high relative humidities. The device can be readily hung near awindow or a lamp and thus be suitably illuminated. A coating of a sensorcomposition of 0.01 mm. thickness or less results in a brilliant opticalreadout.

The reflection-type sandwich of FIG. 4 is a most convenient device toset into the wall of sealed, humidity-sensitive enclosures (such aselectrical junctions and switching boxes where condensation of moistureintroduces serious electrical and corrosion problems). In such aninstallation the humidity sensor has free access to the enclosure air,the circular-type polarizer 20 can be readily installed hermetically ina small aperture in the outer wall of the box, and illuminating beam 5is supplied by room light or by a flashlight.

The graded series of humidity sensors found in the device of FIG. 5 canrange from 15% RH or lower to 85% RH or higher. Though the illustrationshows 10% RH intervals and a 30% through 90% RH range, compositionshaving RH intervals of 5%, 2.5%, or any convenient interval can be used.The RH range can also be selected to suit the needs of the particulardevice. The sensors in the illustration are accompanied by RH legendssince the degradation of stored goods is best gauged in terms of the RHto which the goods are exposed. However, the trigger points of thevarious plaques can be indicated in absolute humidity terms if desired.The sensors of the present invention trigger at aqueous vapor pressuresranging from about 0.1 to 17.5 mm. of mercury at 20° C.

FIG. 6 is a classically simple embodiment of a capacitance-type humiditysensor. As noted before, it offers great accuracy as well as a uniqueand simple mode of optically checking the electrical integrity of thecapacitance-measuring circuitry by noting the capacitance readout at thepoint of optical phase shift.

Ordinary secondary, capacitance-type, polymeric humidity sensorsfunction by sorbing water vapor. Their capacitance vs. RH curve issubstantially non-linear. Thus, complex electrical networks are used inan attempt to correct each hygrometer's readout to a linear one.However, the shape of a polymer's capacitance vs. RH curve changes asthe polymer ages. This complex shift cannot be compensated for. Indeed,although so-called "aqueous humidity calibration salts" are furnishedwith secondary capacitance-type hygrometers in order to aid in fieldcorrection for other types of drift, such field compensation is notsuccessful.

In contrast, the primary, humidity-sensing salts of the presentinvention are highly stable, definite chemical entities. They can bereadily used in a pure, uncompounded form in high precision hygrometersof the capacitance-type, of the optical type, or of a combination type.

As an example, pure cesium benzoylpropionate, in the form of a thin filmin a sensor 3, such as FIG. 6, generates a strictly linear capacitancevs. RH curve over the RH range of 22% through 72%. An explained before,linear extension of this range is readily accomplished. Further, thissimple sensor has the same sensitivity, namely 0.1-0.2 pF/1% RH/sq. mm.of film, as the finest secondary, capacitance-type sensors which havebeen developed over the years. In short, the new technology offers atlow cost a number of unusual features of great importance in thehumidity-sensing field.

There are a few important applications for humidity sensors whereexceptionally rigorous conditions are encountered during their lifecycles. Perhaps the best known example of such unusual circumstances isthe 500° C. cycle to which humidity sensor chips are exposed during thehermetic sealing of military-type ceramic "packages" which houseintegrated circuits. The sensor chips (which have been secondary typesensors until now) are sealed within the package to monitor its interiorhumidity, the electrical connections being brought out through packagepins.

Secondary type capacitive or conductive sensors offer many problemssince the sensor within the hermetically sealed package cannot bestandardized after firing. Thus, there is a real need for primary type,non-drifting sensors such as those of the present invention.Unfortunately, though the organic type sensors of this invention areexceptionally stable, they cannot withstand 500° C. firing. For suchapplications I have found inorganic hydrates of great utility. Typicalof the chemical compounds which may be used in capacitance-type,humidity-responsive devices which may be cycled to elevated temperaturesare these: calcium chloride, barium perchlorate, magnesium perchlorate,calcium sulfate, calcium bromide, and copper sulfate.

Calcium chloride typifies a salt I have found most useful as acapacitance-type sensor since it may be cycled to 500° C. for anextended period and still retain its primary humidity sensingcharacteristics.

I have found that the finely inorganic compounds which are so useful incompounding the organic sensor salts of the present invention (and whichI have discussed before) also are very effective in the compounding ofthese inorganic salts as to secure high sensitivity and freedom fromdeliquescence problems. It is theorized that the sensor coatings ofthese very high surface area materials are only a few molecules thickand so are very responsive. As an example, a 0.25 molar solution ofchemically pure calcium chloride, thickened with from about 0.5 to 4.0percent by weight of a pyrogenic silica (200 sq. meters surface area/g.)forms a most effective coating suspension. The curve of capacitance vs.RH of such a sensing film gradually slopes upward from about 5% RH to15% RH at which point there is a large and almost vertical rise beforeresuming a linear slope. It is an excellent curve with which to trackthe critical 8 to 20% RH range within the "sealed" package so as toeasily detect micro leaks.

I have found that calcuim bromide generates a capacitance vs. RH curvewhich is of the same characteristic shape as that of calcium chloride,FIG. 8c, when compounded with the same silica. However, the large andalmost vertical rise occurs at about 5% RH at 20° C. instead of the 15%RH characteristic of calcium chloride. Further, I have found thatcalcium chloride and bromide may be blended so as to secure capacitancevs. RH curves in which the large rise occurs at intermediate humidities.

In summary, and as has been previously discussed, to create a gradedseries of humidity sensors where optical trigger points vary from 15% RHto 85%, I typically select a sensor salt from the potassium and/orcesium alkali metal salts of benzoylpropionic acid and/orethylbenzoylpropionic acid, stabilize it with an alkali metalperfluoralkyl acid phosphate, complex it with the alkali metal salt of apolymer with repetitive carboxyl groups whose acidic hydrogens have beenreplaced by the alkali metal cations (and which has been complexed witha selected polyvalent cation of a water soluble salt such as copperacetate along with an alkali metal borate), and, if desired, reduce itstrigger humidity by adding a crystal modifier such as cesiumhydroxybutyrate.

I claim:
 1. A combination optical and capacitance humidity-responsivedevice for selectively and directly monitoring optical and electricalchanges in a chemical composition comprising (1) a source ofillumination providing a light beam, (2) a chemical compositioncomprising a lyotropic liquid crystal salt having an organic ringportion that is hydrophobic and a polar head that is hydrophilic andwhich of itself reversibly senses changes in humidity, the combinationbeing triggered optically at a particular water vaporpressure/temperature from an anisotropic and translucent liquid crystalstate characteristic of a first water vapor pressure/temperaturecondition immediately contiguous to said particular water vaporpressure/temperature to an isotropic and transparent liquid statecharacteristic of the particular water vapor pressure/temperature, whileits dielectric constant is changing substantially and reproducibly asthe water vapor pressure/temperature changes, (3) light detecting meansfor detecting observable changes in the brightness and intensity of thelight coming from the composition when the light beam contacts thecomposition, the light from the composition at the first water vaporpressure/temperature being of sufficient brightness and intensity toprovide a visual signal at the first water vapor pressure/temperaturebut not at the particular water vapor pressure/temperature, (4)electrode means insulated from the composition and capacitivelyassociated therewith and means for impressing a voltage across saidelectrode means and (5) electrical capacitance measuring means connectedto said electrode means for measuring changes in the dielectric constantof the composition in response to changes in the water vaporpressure/temperature of said composition.
 2. A device as defined inclaim 1 in which the composition is birefringent at the immediatelycontiguous water vapor pressure/temperature when in a liquid crystalstate and non-birefringent at the particular water vaporpressure/temperature when liquid.
 3. A device as defined in claim 1 inwhich the means for detecting observable changes includes means forilluminating the composition which when translucent at the immediatelycontiguous water vapor pressure/temperature provides light of sufficientintensity and brightness to provide a visual signal.
 4. A device asdefined in claim 1 in which the light detecting means includeselectrical means responsive to the visual signal at the immediatelycontiguous water vapor pressure/temperature and non-responsive to thesignal at the particular water vapor pressure/temperature.
 5. A deviceas defined in claim 1 in which the electrical capacitance measuringmeans is calibrated in percent relative humidity.
 6. A device as definedin claim 1 and wherein the electrical capacitance measuring meansincludes an LED activated by the light detecting means to visuallyindicate that said particular water vapor pressure/temperature has beenreached to thereby confirm the operation of the electronic circuitry inits measurement of both optical and electrical parameters.
 7. A deviceas defined in claim 1 in which the chemical composition is a mixture ofan alkali metal salt of 3-benzoylpropionic acid and a lattice modifierwhich is an alkali metal salt of hydroxybutyric acid.
 8. A device asdefined in claim 1 in which the chemical composition is a mixture of analkali metal salt of 3-benzoylpropionic acid and a lattice modifierwhich is an alkali metal salt of one or more carboxylic acids selectedfrom the group consisting of malonic, tartronic, succinic, levulinic,ketoglutaric, hydroxybutyric, succinamic, and aminobutyric acids.
 9. Adevice as defined in claim 1 in which the chemical composition comprisesa mixture of an alkali metal salt of 3-benzoylpropionic acid and3-(4-ethylbenzoyl) propionic acid.
 10. A device as defined in claim 1 inwhich the saturated aqueous solution of the composition has a vaporpressure in the range of about 0.1 to 17.5 mm. of mercury of 20° C. 11.A device as defined in claim 1 in which the composition is a coating ofapproximately 0.01 mm. thickness on a translucent or transparent,isotropic substrate.
 12. A device as defined in claim 1 in which thechemical composition is a mixture of the alkali metal salts of3-benzoylpropionic acid, of a lattice modifier which is an alkali metalsalt of hydroxybutyric acid, of a lattice stabilizer comprising analkali metal salt of a perfluoroalkyl acid phosphate at a concentrationof 0.1 to 0.3% by weight, on the basis of solids, and of a complexcomprising (a) a water-soluble salt of Cu⁺⁺ at a concentration of 0.4 to5.0 mol percent of the metals present in the chemical composition, (b)an alkali metal salt of polyacrylic acid in a concentration of from 2.0to 8.0% by weight for each 8% of the alkali metal salt of the3-benzoylpropionic acid, and (c) an alkali metal borate in concentrationof from 0.25 to 10.0 mol percent of the metals present in the chemicalcomposition.
 13. A device as defined in claim 1 in which the compositionis the potassium salt of 3-benzoyl propionic acid.
 14. A device asdefined in claim 13 in which the composition is translucent at theimmediately contiguous water vapor pressure/temperature when lesshydrated transparent at the particular water vapor pressure/temperaturewhen hydrated.
 15. A device as defined in claim 13 in which thecomposition is translucent at the immediately contiguous water vaporpressure/temperature when crystalline and transparent at the particularwater vapor pressure/temperature when liquid.
 16. A device as defined inclaim 1 in which the capacitance measuring means includes adjustableelectrical means responsive to the capacitance signal at the immediatelycontiguous water vapor pressure/temperature but non-responsive to thecapacitance signal at the particular water vapor pressure/temperature.17. A device as defined in claim 16 in which the electrical meansincludes control means that is responsive to the capacitance signal forcontrolling electrical power to activate an electrical circuit.
 18. Acapacitance, humidity-responsive device comprising means for providingobservable changes in humidity comprising (1) a chemical compositioncomprising a lyotropic liquid crystal salt having an organic ringportion that is hydrophobic and a polar head that is hydrophilic andwhich of itself reversibly senses changes in humidity, the compositionbeing triggered optically at a particular water vaporpressure/temperature from an anisotropic and translucent liquid crystalstate characteristic of a first water vapor pressure/temperaturecondition immediately contiguous to said particular water vaporpressure/temperature to an isotropic and transparent liquid statecharacteristic of the particular water vapor pressure/temperature, whileits dielectric constant is changing substantially and reproducibly asthe water vapor pressure/temperature changes, (2) electrode meansinsulated from the composition and capactively associated with saidcomposition, and electrical means for impressing a voltage across saidelectrode means, and (3) electrical capacitance measuring meansconnected to said electrode means for measuring changes in thedielectric constant of the composition in response to changes in thewater vapor pressure/temperature of said composition.
 19. A device asdefined in claim 18 in which all elements of the device have beenminiaturized, and the electrical capacitance measuring means formeasuring changes in the dielectric constant of the composition is anintergrated circuit.
 20. A humidity-responsive device as defined inclaims 1 or 18 and wherein the chemical composition comprises one ormore alkali metal salts of an acid of the general formulaR-Aryl-CO-X-(CH₂)_(Y) -CO₂ H, where R is hydrogen, halogen, alkyl,alkoxy, or nitro, where Aryl is phenyl or a connected ring structure ora fused ring structure, where X is a nitrogen or a carbon attached tothe adjacent methylene with either a single or double bond, and where Yis 0 to
 7. 21. A device as defined in claims 1 or 18 in which at leasttwo discrete adjacent areas capacitively associated with the electrodemeans are occupied with compositions comprising a series of compounds ormixtures of compounds which trigger at different water vapor pressuresat 20° C.
 22. A device as defined in claims 1 or 18 in which thechemical composition contains before coating from 10-50% by weight ofsensor solids compounded with from 0.5 to 4.0% by weight of inorganiccompounds of high surface area.
 23. An optical humidity-responsivedevice comprising (1) a source of illumination providing a light beam,(2) means for providing observable changes in humidity comprising achemical composition which comprises one or more alkali metal salts ofan acid of the general formula R-Aryl-CO-X-(CH₂)_(Y) -CO₂ H, where R ishydrogen, halogen, alkyl, alkoxy, or nitro, where Aryl is phenyl or aconnected ring structure or a fused ring structure, where X is anitrogen or a carbon attached to the adjacent methylene with either asingle or a double bond, and where Y is 0 to 7 and which composition ofitself reversibly senses changes in humidity, said composition beingtriggered optically at a particular water vapor pressure/temperaturefrom an anisotropic and translucent phase characteristic of a firstwater vapor pressure/temperature condition immediately contiguous tosaid particular water vapor pressure/temperature to an isotropic andtransparent phase characteristic of the particular water vaporpressure/temperature, and (3) light detecting means for detectingobservable changes in the brightness and intensity of the light comingfrom the composition when the light beam contacts the composition, thelight from the composition at the first water vapor pressure/temperaturebeing of sufficient brightness and intensity to provide a visual signalat the first water vapor pressure/temperature but not at the particularwater vapor pressure/temperature.
 24. An optical humidity-responsivedevice as defined in claim 23 and wherein the connected ring structureis biphenyl.
 25. An optical humidity-responsive device as defined inclaim 23 and wherein the fused ring structure is of the group consistingof naphthalene, acenaphthene, fluorene, anthracene and pyrene.
 26. Adevice as defined in claim 23 and comprising a rectangularly shapedsolid, isotropic material having a cross-section which causes internalreflection of light when such light is directed to its front, at least25% of the device's back side being coated with the composition having athickness of approximately 0.01 mm.
 27. A device as defined in claim 26in which the cross-section of the isotropic material is one of the groupconsisting of semi-circular, semi-annular or trapezoidal.
 28. A deviceas defined in claim 26 and wherein the selected cross-section istrapezoidal and the angles of the trapezoid being selected so as tosecure total internal reflection of light within the material.
 29. Adevice as defined in claim 20 or 23 in which the trigger point of thechemical composition is reduced to a lower water vaporpressure/temperature through the inclusion of a lattice modifier whichcomprises one or more of the alkali metal salts of an acid of thegeneral formula R-CZ-(CH₂)_(Y) -CO₂ H, where R is amino, hydroxyl,carboxyl, or methyl, where Z is hydrogen, hydroxyl, or a keto, and whereY is 0 to
 4. 30. A device as defined in claims 23 or 20 in which thecomposition is optically stabilized through the inclusion of from 1.0 to3.0% by weight, on the basis of solids of a lattice stabilizer whichcomprises a low vapor pressure aryl hydrocarbon from the groupconsisting of: hydrogenated terphenyl, terphenyl, 1,1-di(orho-xylyl)ethane, 1,2,3,4 tetramethyl benzene, 1,2,4,5 tetramethylbenzene, 1,2,3,5tetramethyl benzene, 1,2,3,4,5 pentamethylbenzene, andcyclohexylbenzene.
 31. A device as defined in claims 23 or 20 in whichthe composition is optically stabilized through the inclusion of from0.1 to 0.3% by weight, on the basis of solids, of a lattice stabilizerfrom the group consisting of alkali metal perfluoralkyl sulfonates,carboxylates, and acid phosphates, whose fluorocarbon chains contain 3to 5 carbons.
 32. A device as defined in claims 23 or 20 in which thechemical composition is complexed with from 0.4 to 5.0 mol percent ofthe metals present of a water-soluble salt of a polyvalent cationselected from the group consisting of Cu⁺⁺, Ni⁺⁺, Co⁺⁺, Mn⁺⁺, Al⁺⁺⁺,Zn⁺⁺, and Cr⁺⁺⁺, and in which each 8% of alkali metal sensor salt iscomplexed with from 2.0 to 8.0% by weight of a polymer which hasrepetitive carboxyl groups whose acidic hydrogens have been replaced byalkali metal cations, and in which the chemical composition is complexedwith an alkali metal borate in an amount from 0.25 to 10.0 mol percentof the metals present.
 33. A device as defined in claims 23 or 20 inwhich the composition comprises an alkali metal salt of3-benzoylpropionic acid.
 34. A device as defined in claims 23 or 20 inwhich the composition comprises mixtures of the alkali metal salts of3-benzoylpropionic acid.
 35. A device as defined in claims 23 or 20 inwhich the composition comprises an alkali metal salt of3-(4-ethylbenzoyl) propionic acid.
 36. A device as defined in claims 23or 20 in which the composition comprises mixtures of the alkali metalsalts of 3-(4-ethylbenzoyl) propionic acid.
 37. An opticalhumidity-responsive device comprising (1) a source of illuminationproviding a light beam, (2) means for providing observable changes inhumidity comprising a chemical composition which comprises one or morealkali metal salts of an acid of the general formulaR-Aryl-CO-X-(CH₂)_(Y) -CO₂ H, where R is one of the group consisting ofhydrogen, halogen, alkyl, alkoxy, or nitro, where Aryl is phenyl or aconnected ring structure or a fused ring structure, where X is anitrogen or a carbon attached to the adjacent methylene with either asingle or a double bond, and where Y is 0 to 7 and which composition ofitself reversibly senses changes in humidity, said composition beingtriggered optically at a particular water vapor pressure/temperaturefrom an anisotropic and translucent phase characteristic of a firstwater vapor pressure/temperature condition immediately contiguous tosaid particular water vapor pressure/temperature to an isotropic andtransparent phase characteristic of the particular water vaporpressure/temperature, (3) means to amplify observable changes in thecomposition due to changes in the humidity including a polarizer for thelight beam and an analyzer for the polarized beam that is passed throughthe composition, and (4) light detecting means for detecting observablechanges in the brightness and intensity of the light coming from thecomposition when the light beam contacts the composition, the light fromthe composition at the first water vapor pressure/temperature being ofsufficient brightness and intensity to provide a visual signal at thefirst water vapor pressure/temperature but not at the particular watervapor pressure/temperature.
 38. A device as defined in claim 37 in whichthe device is a transmission type in which the light beam passes throughthe polarizer and the polarized light beam passes through thecomposition, where it is doubly refracted at the first water vaporpressure/temperature, and the emerging refracted beam passes through theanalyzer.
 39. A device as defined in claim 37 in which the device is areflective type, the light beam passes through a circular polarizer andthe composition to a mirror and then being reflected back through thecomposition and the analyzer.
 40. A device as defined in claim 37 andwherein the connected ring structure is biphenyl.
 41. A device asdefined in claim 37 and wherein the fused ring structure is of the groupconsisting of naphthalene, acenaphthene, fluorene, anthracene andpyrene.